Yeah. Okay, first, you need the program Tahaxan and the right .dll files. Let me know if you have trouble finding those. Then, you need DSLazy, or another program capable of unpacking the rom. Then, make a copy of your original rom. This copy is the one you will be hex editing your new sprites into. Once you have it unpacked, put a .narc extension on the file named "6" in A\0\0. Then use DSLazy to repack the NDS_UNPACK, and make a rom with the .narc extension on that file. This won't be playable, but it will be openable in Tahaxan. Alright, the next step is to open your edited ROM in Tahaxan. Go to the A\0\0\6 file. See how it has that .narc extension? That was the point of the first few steps. Open that, and you will find the palette and sprite files for all of the trainer backs. By right clicking on one, you can Extract it from the rom. Make note of the Hex Offset of where that file is located; you're going to need that to Hex Edit your edited sprite back in. Now, when I did this, I was simply replacing the heroine's back with a different one. If you want to replace it with a sprite made from scratch, you'll need to get it in the right format, and I can't help you with that; you're going to have to figure that out on your own. It might work if you used PokeDSPic to insert the back into a Platinum ROM, and then opened THAT rom in Tahaxan, and extracted it once it was in the correct format. Just make sure it's the perfect amount of data: 25648 bytes for the sprite, and 552 for the palette. Anyway, once you have the files you want to insert (both the sprite and the palette,) find the back that you want to replace, and write down the offset for both it and its palette. Now we're going to need a Hex Editor. I use Hex Workshop, but you can probably find another that would work a well. Open up your ORIGINAL rom, from before you added the .narc extension. (This is why we made a copy.) Find the hex offset for what you wrote down, and simply replace that data with your edited data. Make sure you do the same for the palette; usually, the palette's data is immediately after the sprite's data. Also, occasionally the offset given by Tahaxan isn't exactly right; by extracting the data you want to replace with Tahaxan, you can look at it in your Hex Editor and make sure that you replace exactly the right data. Make sure you don't replace anything in the wrong offset; you might screw up some other file. Anyway, once you've replaced the data, you're finished! This all sounds a little complicated, I know, but once you've done it once or twice, you get into the swing of things, and can do it fairly easily.
Oh, one last thing: if I remember correctly, the hero and heroine back sprites are in the .narc file twice. I believe the second one is the one you have to replace, but you can replace both if you want to be on the safe side.
Good luck! Let me know if you need any more help.
